Spring'10 release is just around the corner and here are some features that i found really cool !!

Authenticated Force.com Sites:

The only way of authenticating a Force.com site was by using Portal licenses or by writing your own custom logic to validate logins (I personally felt this was a major drawback). But now, with Spring'10 you have a special user type called "Authenticated Web Site User" with which you can authenticate your force.com site users. So, no more purchasing licenses and i believe this feature is free.

Code Scheduler:Your Apex Code can now be scheduled to run automatically at regular intrevals. This feature is similar to Cronkit(Appexchange), only difference is that it now comes in-buil in Salesforce.com with no package installations.